Question

I have 3-column integer arrays, whose last 2 elements are for sorting. For example

10 0 1

11 0 2

12 1 2

13 0 1

I want them to become:

10 0 1

13 0 1

11 0 2

12 1 2

The arrays are first sorted according to the 2nd column, and then again according to 3rd column.

I have over 3000 rows, so I need something also fast. How can you do this in c++?

Note: The array will be allocated dynamically using the following templates:

template <typename T>
T **AllocateDynamic2DArray(int nRows, int nCols){
    T **dynamicArray;

    dynamicArray = new T*[nRows];
    for( int i = 0 ; i < nRows ; i++ ){
        dynamicArray[i] = new T[nCols];
        for ( int j=0; j<nCols;j++){
            dynamicArray[i][j]= 0;
        }
    }
    return dynamicArray;
}

in main,

int ** lineFilter = AllocateDynamic2DArray(2*numberOfLines,3);

Answer 1

I think this should work:

template<typename T>
struct compareRows {
    bool operator() (T * const & a, T * const & b) {
        if (a[1] == b[1])
            return a[2] < b[2];
        else
            return a[1] < b[1];
    }
};

std::sort(dynamicArray, dynamicArray+nrows, compareRows<int>());

Use a functor to implement the comparison between the rows. The sort will take pointers to the beginning of each row and swap them according to the contents of the rows. The rows will stay in the same places in memory.